As a fuel source to replace fossil fuels, ethanol has attracted attention, and the market size thereof is predicted to be 55 million kiloliters in the year 2010. However, to use ethanol as a fuel, a crude product obtained from a biological raw material such as corn must be distilled and refined, and this must be dehydrated to at least 99.5 wt %.
Conventionally, in dehydrating, a dilute ethanol aqueous solution has been distilled in a distilling column so as to be concentrated to a point close to the azeotropic point of an aqueous ethanol, and then the solution has been dehydrated.
As a method for dehydrating an azeotrope, a method is available in which an entrainer is added to the azeotrope, and dehydration is accomplished by azeotropic distillation. However, the method requires a process in which a three-component azeotrope is distilled, and furthermore, the entrainer must be recovered. Therefore, the method has some drawbacks such as large amount of heat energy being required.
Another method is available in which plural molecular sieve tanks are arranged in parallel, and dehydration is accomplished while these tanks are switched over in a batch mode. This method also has a drawback in that the regeneration of the molecular sieve tank consumes substantial amounts of energy.
Furthermore, there has been known a method in which water is separated from a liquid mixture that is mutually completely soluble by a membrane separation process using the pervaporation method using a membrane separator (Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-124444). The membrane separation process using the pervaporation method has advantages of high separation performance and energy saving in the separation of a liquid mixture that is mutually soluble.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-124444